1. Field of the Invention
The present invention relates to a webbing retractor which is provided with a pretensioner for imparting a tensile force to a webbing at the time of a sudden deceleration of a vehicle as well as a force limiter for preventing a tensile force of a predetermined value or more from being applied to the webbing.
2. Description of the Related Art
Among webbing retractors which are installed in vehicles, there is a type which is provided with a pretensioner for taking up the webbing and applying a tensile force thereto when a large deceleration of a predetermined value or more is applied to the vehicle, so as to restrict a wearer of the webbing without slack (refer to, among others, Japanese Patent Application Laid-Open (JP-A) No. 4-92748). FIG. 6 shows a webbing retractor 300 having such a pretensioner.
With this webbing retractor 300, a hollow cylindrical spool 306 around which a webbing 304 is wound is rotatably supported by a frame 302.
A pretensioning shaft portion 308 which is provided integrally at one end of the spool 306 is inserted loosely in a drum 310. The drum 310 is arranged such that a pair of semicircular clamping portions 312 are connected to each other by a pair of connecting portions 314 which are curved substantially in an S-shape, and the drum 310 as a whole is thus formed in a substantially hollow cylindrical shape. An intermediate portion of a wire 318 having one end connected to a drive plate 316 is wound around the outer periphery of the drum 310. An insertion hole 320 into which the drum 310 is inserted is formed in a central portion of the drive plate 316. A pair of engaging portions 322 provided projectingly on the drum 310 are respectively accommodated in a pair of enlarged-diameter portions 320A formed in the inner periphery of the drive plate 316 defining the insertion hole 320. The drive plate 316 is thus adapted to rotate integrally with the drum 310.
In addition, the other end of the wire 318 is connected to a piston (not shown) inside a cylinder 324. When a deceleration of a predetermined value or more is applied to the vehicle, a gas generator is operated, and the piston is rapidly drawn into the cylinder.
A lock portion (not shown) is provided at the other end of the spool 306, and when a deceleration of a predetermined value or more is detected, this lock portion locks the rotation of the spool 306 in the pulling-out direction of the webbing 304, thereby allowing the rotation only in the taking-up direction of the webbing 304.
Accordingly, when the deceleration of a predetermined value or more is applied to a vehicle body, the wire 318 is pulled, so that the wire 318 is wound tightly around the drum 310, and the connecting portions 314 become deformed, causing the clamping portions 312 to approach each other. The clamping portions 312 thus clamp the pretensioning shaft portion 308, so that the drum 310 is made integral with the spool 306. When the wire 318 is further pulled, the spool 306 is rotated by this tensile force, allowing the webbing 304 to be drawn in.
Meanwhile, a webbing retractor has been proposed which is provided with a force limiter for preventing the tensile force applied to the webbing from assuming a predetermined value or more.
In a webbing retractor (refer to Japanese Patent Application Laid-Open (JP-A) No. 7-47923) which is equipped with a so-called torsion bar-type force limiter, a cylindrical torsion bar capable of being torsionally deformed in the circumferential direction is inserted in the hollow cylindrical spool concentrically therewith, and one end of this torsion bar is fixed to the spool. At the time of a sudden deceleration of the vehicle, the other end of the torsion bar is locked by a lock mechanism. Since one end of the torsion bar is fixed to the spool, when a tensile force acting in the pulling-out direction is applied to the webbing at the time of the sudden deceleration of the vehicle, the torsion bar undergoes torsional deformation to rotate the spool, thereby preventing a tensile load being applied to the webbing from assuming a predetermined value or more.
With the webbing retractor 300, since the arrangement provided is such that the wire 318 is wound tightly around the drum 310 at the time of a sudden deceleration of the vehicle, which in turn causes the clamping portions 312 to clamp the pretensioning shaft portion 308, and the drum 310 rotates together with the spool 306, in the state in which the wire 318 is pulled out by a maximum degree, the webbing 304 cannot rotate further in the pulling-out direction of the webbing 304 due to the tensile force of the wire 318. Accordingly, if the above-described force limiter is simply installed in this webbing retractor 300, it is impossible to torsionally deform the torsion bar sufficiently.